A driver assistance system is an arrangement which supports the driver of a motor vehicle when guiding the vehicle. Known examples are: anti-blocking systems as well as drive dynamic control systems which support the driver in an unstable driving situation by brake intervention and/or motor interventions which operate to stabilize.
Further examples of driver assistance systems are arrangements which hold the speed of the vehicle and/or the distance to the ahead-traveling vehicle within desired limits.
Furthermore, systems are under development which support the driver for a transverse guidance of the vehicle. Such driver assistance systems are also characterized as lane-keeping systems and, as a rule, include sensor means which detect the distance of the motor vehicle to the edges of the roadway. This distance is controlled on the basis of these signals with the aid of an external force steering.
A driver assistance system of this kind is disclosed in German patent publication 198 21 163. According to this publication, the driver assistance system is deactivated from time to time in a controlled manner in specific driving states. A controlled deactivation takes place in a wanted manner differently from a disablement of the driver assistance systems because of a defect. These specific operating states correspond, for example, to situations for which the driver assistance system is not designed and in which the system can therefore not purposefully support the driver.
In order to signalize the deactivation to the driver, German patent publication 198 21 163 mentions also a “touch-sensitive” signalization in addition to optical and acoustic signals. According to this publication, such a signalization results when, because of the absence of a control of the steering by a track maintaining device, a corresponding force feedback announcement is no longer present. Stated otherwise, a touch-sensitive message to the driver as to a deactivated driver assistance system takes place in that a support function on the steering wheel is no longer perceptible. Such a signalization takes place passively without active generation of a touch-sensitive signal.
In general, several cases are conceivable wherein a driver assistance system for transverse guidance support switches off automatically. For example, the lane data becomes lost because no roadway markings are present. Furthermore, unclear lane data can be present, for example, double-white and yellow markings at a roadway construction site. In this case, the driver must immediately recognize the situation and again take over the driving task. When the driver assistance system for transverse driving support is at first active and is then switched off, there are two cases to be distinguished.
In a first case, the vehicle is on an ideal line. As a consequence, the transverse guidance support system outputs no actuating quantity. In this way, there is also no feedback of an actuating variable perceptible in the steering manipulation means which, in general, is a steering wheel. In this case, it can happen that the driver does not notice that the system is no longer available because the feel on the steering wheel without an applied force does not differ from the normal state. Acoustic or optical displays of the system switchoff may not be heard or can be overlooked.
A second case is present when the vehicle is not on the ideal line. In this case, the transverse guidance support system outputs an actuating variable which is perceptible in the steering wheel. When, at this instance, the transverse guidance support system is switched off abruptly, this can lead to dangerous driving behavior especially in the case of an unattentive driver when the driver realizes too late that the transverse guidance support system is no longer available. To avoid abrupt switchoffs, and for a safe and simultaneously comfortable function, one would select a continuous reduction of the additional torque on the steering wheel which results from the transverse guidance support. However, the switchoff can go unnoticed by the unattentive driver. If the driver in this case follows the reduced additional torque, which is still perceptible in the steering wheel, this can lead to a leaving of the driving lane because the still perceptible additional torque is no longer commensurate with the driving situation.